1. Field of the Invention
The present invention relates to a method of automatically sharpening weighting value in an image sharpening process, and more particularly to a method of automatically adjusting weighting values of a sharpening function and avoiding accompanying noise increase simultaneously in an image sharpening process performed in an image processing apparatus.
2. Description of the Related Art
In general, it is well known that, in an image processing apparatus, defocused images take place due to a possible degradation in the MTF (Modulation Transfer Function) of devices used therein.
Heretofore, in conventional picture processing apparatus, in order to correct these defocused images in a picture, a spatial filtering process is applied. This spatial filtering process is elucidated below. Defocused images in pictures are basically caused by possible imperfections of the MTF characteristics. Therefore, the spatial filtering processing is carried out by enhancing the data of relevant pixels by utilizing the data of a plurality of reference pixels in the vicinity of those relevant pixels. Namely, an amount of correction is determined from a difference between a data of the relevant pixel and a sum of data of pixels in the vicinity of the relevant pixel. The correction is achieved by adding this correction value to the data of the relevant pixel.
In general, when such the MTF correction was applied, noises existing in areas wherein the density distribution is flat were also enhanced and hence the Signal/Noise ratio was degraded. Thus this correction procedure eventually produces a picture which gives a noisy feeling. For solving this difficulty, an improved scheme had already been proposed that, when an absolute value of a difference between the data of the relevant pixel and a sum of the data of pixels in the nearest neighbor to the relevant pixel is less than a specified value, that is, by making no MTF correction.
In accordance with the constitution such as described above, however, a problem is left to be solved. The problem is that, (i) when the noise level in the flat portion of the density distribution of a picture is less than the above-mentioned specified value, influence of noise is not certainly included in the relevant pixels, (ii) however, when a contour line and noise are included at the same time in the relevant pixel or reference pixels, and also as a result of this, the difference between the data of the relevant pixel and a sum of the data of nearest-neighboring reference pixels exceeds a specified value, influence of noise is eventually included in the corrected data of the relevant pixel. In such circumstances, there arises an inconvenience that, even for a contour line which is expected to be a smooth line of constant density, level variation is to be include.
Furthermore, there is another inconvenience as follows: when the absolute value of the difference between the data of the relevant pixel and the sum of the data of pixels in the nearest neighbor to the relevant pixel is less than a specified value. Therefore, for cases such that the level of the contour line varies gradually from a high level to a low level, the enhancement process stops after the level becomes below a certain value, and the continuity of contour lines is interrupted.
And, in the conventional prior art, only one common coefficient or weighting values of the MTF was used for both those areas wherein the level difference is large and also for those areas wherein the level difference is small. This also gives another inconvenience that contour lines are enhanced excessively in those areas wherein the level difference on the contour line is large.
Furthermore, there is still another inconvenience. That is, although low-level noises in the high frequency range of the spatial frequency are certainly not enhanced, these noises are to appear on the picture without receiving any correction.
In view of the drawbacks mentioned with the prior art, there is a continued need to develop a new and improved method of image scanner that overcome the disadvantages associated with prior art. The requirements of this invention are that it solves the problems mentioned above.